Snap-through gasket for galvanic cells

ABSTRACT

An electrochemical cell containing a snap-through sealing gasket that has a compressive preloading on a selected area of the snap-through gasket to prevent premature venting of the cell.

FIELD OF THE INVENTION

The invention relates to a compressive preloaded snap-through gasketseal for galvanic cells and more particularly to a seal means forpreventing premature release of gas pressure from inside the cells.

BACKGROUND OF THE INVENTION

Galvanic cells may generate large quantities of gas under certainconditions during use. Since these cells are required to be tightlysealed at all times in order to prevent loss of electrolyte by leakage,high internal gas pressures may develop. Such pressures may causeleakage, bulging or possible explosion of the cell if not properlyvented. If a vent valve is employed, it may be resealable in order toavoid drying out of the electrolyte over the life of the cell and toprevent ingress of oxygen from the atmosphere which can cause wastefulcorrosion of the anode.

In the past several different types of resealable pressure relief ventvalves have been used for releasing high internal gas pressures frominside a sealed galvanic cell. One type of valve that has been commonlyused consists basically of a valve member, such as a flat rubber gasket,which is biased into a sealing position over a vent orifice by means ofa resilient member, such as a helical spring. The resilient member orspring is designed to yield at a certain predetermined internal gaspressure so as to momentarily relieve the seal and allow the gas toescape through the vent orifice.

Another type of resealable vent is disclosed in U.S. Pat. No. 3,451,690to Richman issued on Dec. 10, 1968. In this vent, a flat elastomericseal gasket overlies the vent opening and is retained in place by aresilient terminal cap on the top of the cell. This vent operates inbasically the same manner as the vents previously described.

In U.S. Pat. No. 3,664,878 to Amthor issued on May 23, 1972, aresealable vent is disclosed which comprises a resilient deformable ballof elastomeric material positioned to overlie a vent orifice providedwithin the cell's container. A retainer means is positioned over theresilient ball for maintaining the ball in place over the vent orificeand in contact with a valve seat provided around the peripheral edgeportions of the vent orifice and for compressing and deforming theresilient ball into a flattened configuration forming a normallyfluid-tight seal between the flattened ball and the valve seat. Theresilient ball is capable of undergoing further temporary deformationupon the buildup of a predetermined high internal gas pressure insidethe container so as to momentarily break the seal and allow gas toescape through the vent orifice.

Alternates to high pressure resealable vent means are fail safe ventingsystems as illustrated in U.S. Pat. Nos. 3,218,197 and 3,314,824.Specifically in the '197 patent a venting means is disclosed in whichthe seal gasket has a thin section that will rupture or "blow-out" at apredetermined high pressure buildup within the cell. The difficulty withthis type of venting means is that for small diameter cells it isdifficult to obtain consistency in the thickness of the "blow-out"section of the gasket using conventional manufacturing techniques. Inthe '824 patent a puncture-type safety seal is disclosed which comprisesa spring washer positioned within the cell's seal assembly and aradially acting toothed puncture washer. The teeth of the washer sliderelative to the spring washer when the spring washer is subjected tointernal pressure so that at a predetermined gas pressure buildup, theteeth of the washer will puncture the seal's gasket thereby providing avent passage. This type of venting means requires several componentparts, is rather expensive to make and assemble, and is not suitable forsmall diameter cells.

U.S. Pat. No. 4,079,172 discloses sealed galvanic dry cells having atleast one gas venting passage disposed at the interface of the topsurface of the cover and then curled over the rim of the container. Thepassage is defined as being a recess in the cover disposed below thecurled over rim and/or a notch in a portion of the curled over rim.

U.S. Pat. No. 5,227,261 relates to electrochemical cells which arecomprised of a seal member that has a centrally located cylindrical hubjoining a base, which base has a ventable diaphragm portion and anonventable diaphragm portion, which hub has an end extending above thebase and an end extending below the base, into which a current collectoris inserted in an interference fit with the end extending above thebase, which ventable diaphragm portion joins the hub at an interfaceforming an arc of between about 135 degrees and 250 degrees, and whichinterface is the thinnest portion of the base.

U.S. Pat. No. 4,255,499 relates to a galvanic cell in which a first arcportion between about 150° and 190° of the top peripheral edge of thecontainer is turned or curled over the container's closure means so thatwhen a predetermined high pressure build-up develops in the cell, thepressure will cause the closure means to tip or tilt about the diametralaxis formed between the first arc portion and the remaining second arcportion so as to produce a vent passage at the interface of thecontainer and the closure means proximal the remaining second arcportion of the top peripheral edge of the container.

As discussed above, resealable, high pressure relief vent valves aregenerally bulky and/or difficult to incorporate into a cell assembly,bulky type of blow-out safety vents are not suitable for small cellapplications, while low pressure vent means for some cell systems maynot adequately and sufficiently prevent loss of electrolyte throughleakage or prevent ingress of oxygen from the atmosphere which couldcause wasteful corrosion of the anode.

It is, therefore, an important-object of this invention to provide acompact and economical pressure vent for use in a galvanic cell thatwill effectively occupy minimum volume so that maximum volume of thecell can be used for the active components of the cell.

Still another object of this invention is to provide a predeterminedpressure vent using a snap-through gasket for galvanic cells which iseasy to produce and inexpensive to manufacture.

Another object of the present invention is to provide a vent whichprevents premature release of gas pressure from within a galvanic cell.

The foregoing and additional objects will become more fully apparentfrom the following description and the accompanying drawings.

SUMMARY OF THE INVENTION

The invention relates to an electrochemical cell comprising an anodeelectrode, a cathode electrode and an electrolyte contained in acontainer, said container having an open end and a closed end andwherein the open end of the container is sealed with a gasket, saidgasket comprising a base member having an upstanding peripheral wall anda centrally located upstanding wall forming a hub defining an opening;said base member having a first segment extending radially inward fromthe upstanding peripheral wall to a second segment disposedsubstantially in parallel to the upstanding wall of the hub, said secondsegment extending to a third segment disposed between the second segmentand the upstanding wall of the hub, said third segment having a bowedsurface facing the closed end of the container and having a notch,preferably on the inner surface of the third segment, and the bowedthird segment provided with a compressive preload force in at least aportion of the area of the third segment, preferably between 10° to360°, adjacent to the upstanding wall of the hub that will delay theonset of tensile stresses at the area adjacent the hub caused by anypressure buildup of gasses produced in the cell, so that prematureventing of the cell can be prevented.

Preferably the cross-sectional area of the third segment adjacent theupstanding wall at the hub should have a thickness of between 1/8 and3/4 of the thickness of the third segment between the second segment andthe notch. The reduced area adjacent to the hub of the gasket will be incompression and therefore, absorb the initial low pressure of gasbuildup within a cell, so that premature venting of the cell can beprevented. The thickness of the cross-sectional area adjacent to thefirst segment and the second segment has a thickness preferably betweenabout 11/4 and 21/2 times the cross-sectional thickness of the firstsegment. The cooperation of the first segment and the second segmentprovides a hinge type means so that radial forces applied to theperipheral wall of the gasket will be transferred via the first segmentto the second segment, which in turn will cause transfer of force to thethird segment. This force to the third segment will cause the thirdsegment to bow inwardly due to the more flexible characteristics of thethird segment provided by the notch in the internal surface of the thirdsection. The notch or groove is provided to make the third section moreflexible and will bow inwardly during the redrawing and/or crimping ofthe container onto the gasket. The bowing of the third section willprovide a compressive force on the area of the third section adjacent tothe wall of the hub. After the final assembly of the cell, the thirdsegment of the gasket will be in a compressive preloading in the ventcorner or area adjacent to the hub that will delay the onset of tensilestresses upon the gasket. The gasket will be able to tolerate apredetermined pressure buildup within the cell to prevent prematureventing but upon a high pressure buildup within the cell, the thirdsection of the gasket is designed to provide a snap-through feature sothat the third segment will snap from a convex bow design facing theclosed end of the container to a convex bow design facing the open endof the container. The novel snap-through feature of the gasket providesa construction that would occupy minimum internal volume due to theminimum height of the gasket so that maximum internal volume can be usedfor the active components of the cell. Thus, with the novel constructionof the gasket of this invention, the cell pressure on the gasket can beincreased without the venting of the cell. This feature can beaccomplished with a combination of compressive preloading and thecreation of the snap-through behavior of the third segment of thisgasket.

The gasket of this invention is constructed with several segments thatcooperate during the assembly of the cell to place a compressive preloadin a segment of the gasket that will delay the onset of tensile stressesin the vent area of the gasket and therefore prevent premature ventingof the cell. It has been found that using the novel gasket of thisinvention, the onset of tensile stresses can be prevented, if desired,up to about 200 psi internal pressure for C-size cells. Cell closing mayincrease compression in the reduced area of the gasket adjacent the hub.

The purpose of compressive preloading is to eliminate tensile stress asat the inside angle formed by the junction of the third segment and thewall of the hub under normal operating pressures. In the assembled cellthe area of the gasket adjacent to the hub is in compression. As theinternal pressure is increased, the inside angle, defined as the anglebetween the area adjacent the hub and the wall of the hub, passesthrough 90 degrees.

The thickness of the reduced area in the third segment is preferablybetween 1/8 to 3/4 times the cross-sectional average thickness of thethird segment disposed between the notch and the second segment.

The upstanding wall of the hub can be provided with a flange toaccommodate the centrally disposed edge of the cover (inner cover) of acell. Also the upstanding peripheral wall of the gasket could beprovided with a flange to accommodate the peripheral edge of the innercover. If desired, a rib or rim could be disposed on the upstandingperipheral wall of the gasket to secure the peripheral edge of the innercover onto the first and second segments of the gasket.

Preferably, an inner cover having an opening to accommodate the hub isdisposed within the peripheral wall and is secured between theperipheral wall and the hub of the seal assembly.

Preferably, the gasket in conjunction with the cover are secured to thecontainer by redrawing and/or crimping the rim of the container againstthe gasket and the cover. In this embodiment, the gasket which isgenerally a nonconductive synthetic material, will act as an insulatoras well as a barrier to air and moisture and will be strong enough tomaintain its sealing after it has been physically abused such as bydropping or exposure to vibration and/or subject to extreme variationsin temperature and/or humidity and/or abuse charging.

Cylindrical alkaline cells are generally comprised of a containmentmeans and the components located within the containment means. Thecontainment means comprises an elongated container, that is open on oneend, and a vent assembly. The assembly is made up of a terminal coverplate; an elastomeric gasket; and an inner cover and current collector.The members of the vent assembly are inserted into the open end of thecontainer thereby sealing the container. The rim portion of thecontainer is then redrawn and/or crimped inwardly to form a seal.

The sealing gasket of this invention comprises a material selected withconsideration given to its stability in the presence of the electrolyte,its resiliency, and its resistance to cold flow. Suitable polymericmaterials are selected from the group consisting of nylon,polytetrafluoroethylene, fluorinated etheylene-propylene,chlorotrifluoroethylene, perfluoro-alkoxy polymer, polyvinyls,polyethylene, polypropylene, polystyrene and the like. Other suitablematerials would be recognizable by one skilled in the art. In someapplications, additional precautions can be used in conjunction with thegasket of this invention to provide a more effective seal, such ascoating the flange of the gasket surfaces with an adhesive agent such asa fatty polyamide resin or asphalt. The sealing gasket of this inventionis suitable for production techniques such as injection molding. Theconfiguration of the surfaces of the gasket flanges is well suited forease of removal from dies, punches and the like. Preferably the gasketwould be nylon. The terminal cover should be made of a conductivematerial that will not corrode or otherwise deteriorate when in contactwith the materials of the cell. The container for the cell could be madeof stainless steel, iron, nickel, nickel-plated steel, or some otherconductive material.

The invention also relates to a method for making a galvanic dry cellhaving a pressure vent closure with a safety feature that preventspremature venting of a cell which comprises the steps:

(a) placing within a conductive container a separator, an anode, acathode, and an electrolyte;

(b) preparing a gasket comprising a base member having an upstandingperipheral wall and a centrally located upstanding wall forming a hubdefining an opening, said base member having a first segment extendingradially inward from the upstanding peripheral wall to a second segmentdisposed substantially parallel to the upstanding wall of the hub, saidsecond segment extending to a third segment disposed between the secondsegment and the upstanding wall of the hub, said third segment beingbowed so that it faces the closed end of the container and having anotch, and the bowed third segment being provided with a compressivepreload force in the area of the third segment adjacent to theupstanding wall of the hub that will delay the onset of tensile stressesat the area adjacent the hub caused by any pressure buildup of gassesproduced in the cell, so that premature venting of the cell can beprevented;

(c) assembling an inner cover, the gasket and a current collector intothe container; and

(d) reducing the top edge portion of the container over the peripheralwall of the gasket onto the inner cover to provide a sealed cell, andwherein said area adjacent the hub is in a compressive preloading stressstate. As used herein, the term reducing the top edge portion of thecontainer shall mean a redrawing step, a crimping step or any other stepthat will impart a compressive force on the reduced thickness of thegasket.

The notch in the third segment is preferably located between 1/4 and 3/4the length of the third segment and more preferably between 1/3 to 2/3of the length of the third segment.

DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent from the followingdescription thereof when considered together with the accompanyingdrawings which are set forth as being exemplary of the embodiments ofthe present invention and is not intended in any way to be limitativethereof and wherein:

FIG. 1 is a partial cross-section of a gasket for use in a cell.

FIG. 2 is a sectional elevation of the gasket means of FIG. 1 shownafter assembled in a galvanic cell.

FIG. 3 is a partial cross-section of a deformed gasket after beingexposed to high internal pressure.

Referring to FIGS. 1-3, there is shown gasket 1 comprising a base member2 with an upstanding peripheral wall 4 and an upstanding centrallylocated cylindrical hub 6. Disposed in the base member 2 between theupstanding peripheral wall 4 and hub 6 is a first segment 8, a secondsegment 10, and third segment 12. The angle A formed by first segment 8and second segment 10 is shown as about 70°. A flange 38 is shown and isdesigned to support inner cover 19, as shown on FIG. 2. As shown in FIG.1, the thickness of section 11 is about 11/2 times the thickness of thefirst segment 8. FIG. 2 shows the gasket of FIG. 1 assembled in agalvanic cell 20. The inside angle B as shown in FIG. 1 is about 80°formed between the reduced area segment 18 and the wall 7 of hub 6. Anotch 15 is shown in the third segment 12 of the gasket 1. This notch 15is provided to make the third section 12 more flexible and will bowinwardly during the redrawing or crimping of the container onto thegasket. The bowing of the third section 12 will provide a compressiveforce on the reduced thickness portion 18 of the third section 12adjacent to the hub.

FIG. 2 shows an assembled alkaline manganese dioxide-zinc cell 20comprising container 22 having disposed therein an anode mix 24 andcathode 26 separated by a separator 28. The anode mix 24 could compriseparticulated zinc with a gelling agent and an electrolyte, such asaqueous potassium hydroxide. The cathode could comprise manganesedioxide and a conductive material, such as graphite. As shown in FIG. 2,the gasket 1 of FIG. 1 is positioned within the open end of thecontainer 22 where the gasket 1 rests on the cathode 26. If desired, alayer of sealant may be disposed at the interface of the seal and thecontainer. Upon inserting gasket 1, cover 19 and current collector 30 incontainer 22, current collector 30 made good electrical contact in theanode mix 24. Once the gasket 1 is seated within container 22, and thecover 19 is seated within gasket 1, a terminal cover 32 is disposed overthe current collector 30, and then the annular edge segment 34 ofcontainer 22 is radially compressed and crimped against the gasket 1,cover 19, cover 32, thereby radially sealing the gasket 1 to the openingof the container 22.

The inner cover 19 is designed to seat within seal gasket 1 on flange 38of wall 7 of hub 6. An opening 35 is disposed in inner cover 19 and anopening 37 is disposed in terminal cover 32 to permit the internalpressure to vent.

As shown in FIG. 2, the reduced area of the third segment 12 of thegasket 1 is put under a compressing stress during the redrawing and/orcrimping step. This preloading stress on the gasket delays the onset oftensile stresses in the reduced area to prevent premature venting ofcell. FIG. 3 shows the gasket 1 of FIGS. 1 and 2 after being subjectedto a high pressure force from within the cell that invert the thirdsegment 12 outwardly and places the reduced area 18 in tension. If thepressure buildup is severe, the gasket will snap through to permitventing.

It is to be understood that other modifications and changes to thepreferred embodiment of the invention herein shown and described canalso be made without departing from the spirit and scope of theinvention.

What is claimed:
 1. An electrochemical cell comprising an anodeelectrode, a cathode electrode and an electrolyte contained in acontainer, said container having an open end and a closed end andwherein the open end of the container is sealed with a gasket, saidgasket comprising a base member having an upstanding peripheral wall anda centrally located upstanding wall forming a hub defining an opening;said base member having a first segment extending radially inward fromthe upstanding peripheral wall to a second segment disposedsubstantially in parallel to the upstanding wall of the hub, said secondsegment extending to a third segment disposed between the second segmentand the upstanding wall of the hub, said third segment having a bowedsurface facing the internal container and having a notch, and the bowedthird segment provided with a compressive preload force in at least aportion of the area of the third segment adjacent to the upstanding wallof the hub that will prevent premature venting of the cell.
 2. Theelectrochemical cell of claim 1 wherein the notch is located between 1/4and 3/4 of the length of the third segment.
 3. The electrochemical cellof claim 1 wherein an angle of less than about 90° is formed between thearea of the third segment adjacent to the hub and the upstanding wall ofthe hub.
 4. The electrochemical cell of claim 1 wherein the thickness ofthe area of the third segment adjacent to the hub is between about 1/8to about 3/4 times the thickness of the area disposed between the notchof the third segment and the second segment.
 5. The electrochemical cellof claim 1 wherein the upstanding wall of the hub has a flange.
 6. Theelectrochemical cell of claim 1 wherein a cover has an opening toaccommodate the hub and is disposed within the upstanding peripheralwall of the gasket and the upstanding wall of the hub.
 7. Theelectrochemical cell of claim 1 wherein the gasket is made of materialselected from the group consisting of nylon, polytetrafluoroethylene,fluorinated etheylene-propylene, chlorotrifluorethylene,perfluoro-alkoxy polymer, polyvinyls, polyethylene, polyproplene andpolystyrene.
 8. The electrochemical cell of claim 1 wherein the anode iscomprised of zinc, the cathode is comprised of manganese dioxide, andthe electrolyte is potassium hydroxide.
 9. The electrochemical cell ofclaim 2 wherein the anode is comprised of zinc, the cathode is comprisedof manganese dioxide, and the electrolyte is potassium hydroxide. 10.The electrochemical cell of claim 3 wherein the gasket is made ofmaterial selected from the group consisting of nylon,polytetrafluoroethylene, fluorinated etheylene-propylene,chlorotrifluorethylene, perfluoro-alkoxy polymer, polyvinyls,polyethylene, polyproplene and polystyrene.
 11. The electrochemical cellof claim 4 wherein the gasket is made of material selected from thegroup consisting of nylon, polytetrafluoroethylene, fluorinatedetheylene-propylene, chlorotrifluorethylene, perfluoro-alkoxy polymer,polyvinyls, polyethylene, polyproplene and polystyrene.